Enum ordinals -> tableswitch

[tpolecat]

Just following up on a conversation @odersky and I had at SBTB … I was wondering if we could use enum ordinals to emit tableswitches for eligible match expressions. I have had cases where I had to reorder cases as a performance optimization and something like this would have been really helpful.

[smarter]

We don't necessarily need to do the same, but for reference here's how Java does it: http://www.benf.org/other/cfr/switch-on-enum.html, it's complicated because it's designed to survive reordering and addition of fields to the enum.

[He-Pin]

This is a must done,take a look at :

https://github.com/akka/akka/pull/25982/files#diff-ac929e6fd84743df4b6e214b5499d644R423

for why...

[smarter]

@hepin1989 without benchmark results you won't convince us that it's a "must done"

[He-Pin]

Same patterns in scala future,zio, etc

[bishabosha]

and we can't rely on mima because definitions are order sensitive

[nmichael44]

There is some discussion this issue that may help whoever picks this up in the now closed-as-duplicate ticket: #23314

[som-snytt]

Even Kotlin does this.

Most damning of all.

[nmichael44]

@som-snytt 😃 I didn't really mean it as a slight for the Kotlyn compiler. I just meant, it's a newer language so you wouldn't expect the compiler to be that mature and contain this kinds of optimizations.